Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) are a type of transistor having a layer of dielectric between a conductive gate and a semiconductor region. MOSFETs may be designed to operate in enhancement mode or depletion mode. Enhancement mode MOSFETs operate by creating a conductive channel through inversion of the semiconductor surface underneath the gate, that is, by applying a voltage to the gate electrode and creating a region in the semiconductor where the concentration of “minority” carriers (holes in the case of n-type semiconductors, and conduction electrons in the case of p-type semiconductors) is increased until it exceeds the equilibrium concentration of “majority” carriers (by contrast, a depletion mode device operates by applying a voltage to a gate, and reducing the number of carriers to a value lower than the equilibrium value in an already present conductive channel). The conductive channel so created typically extends laterally or vertically between the source and the drain of the device.
Power MOSFETs are a type of MOSFET designed to handle high voltages and/or high currents. In one type of power MOSFET—a double diffused MOSFET (DMOS)— the body and source regions are diffused from the same edge. DMOS devices may be either lateral or vertical devices, depending on whether the current flow between the source and drain is lateral or vertical, respectively. Vertical DMOS technology is used to fabricate a variety of device types, including high voltage and high current transistors and IGBTs (Insulated Gate Bipolar Transistors). The large majority of these vertical DMOS devices are n-channel rather than p-channel devices, due to the lower on-resistance or voltage drop per unit area afforded by n-channel devices as compared to p-channel devices. This lower on-resistance or voltage drop per unit area is a result of the higher mobility of conduction electrons in silicon compared to that of holes.
As with other MOSFETs, vertical DMOS devices may be enhancement mode or depletion mode devices. In an n-channel enhancement mode device, the threshold voltage of the device—that is, the voltage required to be applied to the gate in order to create a conductive channel between the source and the drain, which is also the voltage necessary to effect inversion—is typically chosen to be sufficiently positive with respect to the source voltage to allow the device to be fully “off” with 0 volts present between the gate and the source. In an n-channel depletion mode device, by contrast, the threshold voltage is typically chosen to be sufficiently negative with respect to the source voltage to allow the device to be fully “on” with 0 volts present between the gate and the source. In depletion mode n-channel devices, the conductive channel in the device is typically formed either by introducing n-type dopant at the surface of the body region which produces a permanent channel region with no gate-to-source voltage, or by implanting permanently charged ions into the gate dielectric which induce a channel at the surface of the underlying body region when there is no gate-to-source voltage.
A conventional vertical DMOS is illustrated in FIG. 1. Devices of this type are described, for example, in R. Locher, “Introduction to Power MOSFETS and Their Applications”, National Semiconductor Application Note 558 (December 1988). The device 1 consists of an n+ substrate 2, on one surface of which is disposed an epitaxial layer 3 and on the other surface of which is disposed a metal layer 5 which serves as a drain contact having a drain terminal 6. A deep body region 7 is in electrical contact with a source and body terminal 4 and is formed in the epitaxial layer. In the device illustrated, the shallower diffused region 10 is a p body region and the deeper diffused region is a p+ body region. A portion 13 of the p body region extends underneath the gate between the n+ source region 9 and the drain region 21 and is capable of undergoing inversion to form a channel.
A conductive polysilicon gate 15 is disposed over the channel. The gate is surrounded by a dielectric material 17 (typically SiO2). The portion of this dielectric material below the gate is referred to as the gate dielectric. A layer of source and body metal 19 is disposed over the gate and the epitaxial layer, but in contact with just the source and body regions.
When the gate is biased positive with respect to the source and there is an applied drain-to-source voltage, the holes in the p-type body region 13 are repelled away from the gate area and conduction electrons are drawn towards it, thus inverting the p-type body regions underneath the gate. This gate-to-source voltage creates a path or channel so that carriers from the source can flow to the drain region at the surface below the gate, and then vertically through drain region 21, and to the n+ substrate 2. The vertical geometry of these devices allows lower on-state resistances for the same blocking voltage and faster switching than is possible in comparable lateral MOSFETS.
The threshold voltage of both enhancement mode and depletion mode DMOS devices varies from one device to another. The degree of threshold voltage variation across a group of devices is determined by a variety of manufacturing variables, including the exact doping profile in the body region, the gate dielectric thickness and composition, and the composition of the gate conductor. In typical switching applications, variations in the manufacturing threshold voltage do not present a problem with respect to enhancement mode DMOS devices, because the drive signal is chosen to turn the device fully “on” or fully “off”. However, while depletion mode DMOS devices may be employed in a similar manner, the current verses voltage characteristics of depletion mode DMOS devices (see FIG. 2) for an n-channel DMOS transistor also allows them to be used in a two terminal configuration in applications that require a specific current with zero volts between the gate and the source. Only two terminals are required in such a configuration, because the gate is electrically connected to the source. Some of these applications are described, for example, in S. Ochi, “Semiconductor Current Regulators Protect Circuits”, PCIM, Vol. 26, No. 1, p. 63 (January 2000). In many of these applications, the ability to provide a specific current (within the limits of design specifications) is essential to their use. However, due to the above noted variations in threshold voltages of DMOS devices, the current that flows with zero volts between the gate and source may vary significantly even among devices manufactured using the same process flow.
There is thus a need in the art for depletion mode MOS-gated devices having a threshold voltage that may be adjusted so as to provide a desired current flow with zero volts between the gate and the source. There is also a need in the art for methods for making such devices and for adjusting the threshold voltages thereof. These and other needs are met by the present invention, as hereinafter described.